A wireless device (e.g., a cellular phone or a smart phone) may include a transmitter and a receiver coupled to an antenna to support two-way or multiple-way communication. The transmitter may modulate a radio frequency (RF) carrier signal with voice and/or data to obtain a modulated signal, amplify the modulated signal to obtain an output RF signal having the proper output power level using one or more power amplifiers, and transmit the output RF signal via the antenna to a base station.
Inductors are an integral part of power amplifiers used for wireless communication. For example, inductors can be used in tank circuits, as chokes, etc. When integrating inductors into devices board-level reliability is a major concern. For example, concerns have been raised about the possible effects of solder voids, temperature cycling, mechanical vibration, electromigration, etc., on board-level reliability.
One traditional solution is to implement one or more inductors on a chip using surface mount technology (SMT). This solution requires the inductors to be mounted external to the chip, however. One consequence of this solution is that the choke inductor in the power amplifier is removed and replaced with the SMT inductor. Replacing the choke inductor with the SMT inductor causes the power amplifier to have a reduced noise margin.
Another traditional solution is to embed the inductors in a package substrate. This solution has challenges as well. For example, embedding inductors in a package substrate results in increased packaging costs. Additionally, embedding inductors in a package substrate results in a larger package because of the extra space needed for the inductors.
Still another traditional solution is to implement the inductors on a laminated substrate using land grid array (LGA) technology. This implementation uses a spiral inductor embedded in the substrate using solder balls on the substrate. FIG. 1 depicts a conventional substrate forming a die 102 for implementing an inductor. The die 102 includes several solder balls 104a-104z. When an inductor is implemented on the die 102, some of the solder balls have to be depopulated. For example, FIG. 1 shows (using dotted lines) that solder balls 104k, 104l, 104p, and 104q have to be depopulated for optimum performance of the inductor. If some of the solder balls are depopulated, however, density requirements for solder balls may be violated. For example, to meet board level reliability standards thermal and/or mechanical requirements must be met when there are fewer solder connections between the die 102 and a printed circuit board (PCB).
The alternative is to leave all solder balls on the die 102. If the solder balls 104k, 104l, 104p, and 104q are not depopulated from the die 102, the solder balls 104k, 104l, 104p, and 104q are said to “float” because they are not electrically coupled to the die 102 when the inductor is integrated on the die 102. Unfortunately, floating solder balls cause high eddy currents, which results in lowered inductance of the inductor, higher direct current (DC) resistance (Rdc) for the inductor, and low quality factor (Q) of the inductor.
Also, the values for L, Rdc, and Q are subject to change due to process variations. If the values for inductance vary, there needs to be a way to compensate for the variations, such as by including a variable capacitance in the package. This added component causes the bill of materials to increase, causes the cost of manufacturing to increase, and causes the size of the package to increase.
Thus, improved apparatuses and methods for implementing an inductor, in a power amplifier, for example, are needed.